This invention relates generally to frequency conversion methods and apparatus, and more specifically to low phase noise frequency conversion methods and apparatus particularly suitable for reception and transmission of digital signals including multi-carrier modulated signals.
The European Broadcasting Union (EBU) European Telecommunications Standards Institute (ETSI) Joint Technical Committee (JTC) has published a draft standard EN300 744 v. 1.2.1 (1999–01) for digital television broadcasting, specifying the framing structure, channel coding, and modulation for digital terrestrial television. This standard functionally defines the equipment needed for adaptation of a baseband TV signal from an output of an MPEG-2 (Motion Picture Experts Group-2) transport multiplexer to the terrestrial channel characteristics. ODFM (Orthogonal Frequency Division Multiplexing) with concatenated error correction coding is specified. To achieve the conversion of an MPEG-2 transport stream to an ODFM signal, transport multiplex adaptation and randomization is applied to the input signal. Outer Reed-Solomon (RS) and outer convolutional interleaving is then applied. Next, an inner coding, for example, a punctured convolutional code, is applied with inner interleaving. Finally, a mapping and modulation step is performed, followed by ODFM transmission. The standard allows for either 8, 7, or 6 MHz channel spacings, and both a “2K mode” and an “8K mode.” The “2K mode” is useful for single transmitter operation and small single frequency networks (SFNs) with limited transmitter distances, while the “8K mode” is suitable for both signal transmitter operation and for small and large SFN networks. Different levels of quadrature amplitude modulation (QAM) and different inner code rates are allowed.
The standard is defined in the context of broadcast television transmission. However, the apparatus and process for the conversion of MPEG-2 transport streams into broadcast-compatible digital television coding would be useful for many applications. For example, it would be desirable to provide methods and apparatus for a portable transport stream adapter that can be used for portable television equipment of all kinds, such as electronic news gathering (ENG) equipment. It would also be desirable to provide improved modulation error rate performance, better modulation shoulders and immunity to phase noise, and improved timing accuracy for such equipment.
A digital television system implementing the ETSI standard for digital television broadcasting further requires the conversion of the OFDM signal from a baseband signal to an IF signal and from an IF signal to an RF signal. This latter frequency conversion typically results in the introduction of significant phase noise partially attributable to divider noise. In an IF to RF frequency converter, the input IF signal is mixed with two oscillator signals to upconvert the IF signal to an RF signal. The frequency shift corresponds to the difference between the frequencies of the two oscillators. The oscillator signals are synthesised based upon an external reference signal. The accuracy of the oscillator signals is controlled by the comparing the external reference with the oscillator signal divided by an appropriate factor. This comparison results in a error signal that can be used by the synthesiser to adjust the oscillator signal. The operation of dividing the oscillator signal introduces phase noise that becomes amplified and introduced into the oscillator signal and that then corrupts the output RF signal. A further difficulty is that the resolution of a frequency converter wherein the oscillators are controlled by frequency synthesizers is limited by the step size of the frequency synthesizers.
It would be desirable to provide an apparatus and method of frequency conversion, possible of wide application, with improved phase noise performance and greater frequency resolution.